Which type of corrosion occurs on vehicles?

• A. Filiform corrosion
• B. Galvanic corrosion
• C. Uniform corrosion
• D. Pitting corrosion

Answer: (A)

Filiform corrosion is corrosion that starts beneath a painted coating and grows as worm-like filaments along the coating–metal interface. On vehicles, this shows up when the paint or primer has defects—pinholes, scratches, or edges where moisture can enter. Once a tiny anodic site forms at the coating interface, an electrochemical cell drives current under the coating, and the corrosion front advances outward, creating those threadlike tracks under the coating. This pattern is distinct from uniform corrosion (which would thin the metal evenly), pitting (localized pits), or galvanic corrosion (dissimilar metals in direct contact in the presence of an electrolyte). In automotive coatings, filiform corrosion is commonly observed when coating integrity is compromised, making it the best fit for the scenario.